1. Field of the Invention
The present invention relates generally to the field of systems for fracturing wells. More specifically, the present invention discloses a propellant cartridge for fracturing a well that includes restrictor plugs above and below the propellant charge.
2. Statement of the Problem
Propellants generate high pressure gases that can be used to initiate and extend fractures in hydrocarbon-producing formations. These gases are produced when propellants are burned. The gases form at high temperature and pressure, and are subsequently cooled and condensed or otherwise lost to their surroundings. Conservation of energy tells us that some of the chemical bond energy of the propellant is converted to useful energy, but much of the energy is lost to the following: (a) up the well bore in the form of inertial displacement of the fluid column, which is a function of the fluid and casing characteristics; (b) up the well bore in the form of compression of the fluid column, which is a function of the fluid characteristics; (c) down the well bore in the form of compression of the fluid column, also a function of the fluid characteristics; (d) to the well casing and formation, in the form of expansion and contraction of the well casing and surrounding formation from the well bore pressures; (e) heat loss, in the form of heating the fluids, conversion to a vapor state, and heat loss to the casing and surrounding well formation; and (e friction losses as the gas vapors and fluid are pushed from the well bore into the formation, both through the well casing perforations and into the fractures which exist and are being propagated in the formation.
The more energy that can be directed to the formation to minimize these losses, the more useful work will be done on the formation. “Useful work” is hereby defined as that energy which extends fractures or cleans up existing fractures and perforations, allowing more of the hydrocarbons to flow back into the well bore. The problem is, therefore, to direct as much available energy as possible into the formation, in the form of pressurized gas, without damaging the well casing and surrounding formation.
Conventional packers have been employed in the past in hydraulic well fracturing and in using propellants to fracture wells. A packer typically is lowered to a desired depth in the well and actuated to completely block the well bore above or below the region to be fractured. U.S. Pat. No. 5,295,545 (Passamaneck) shows an example of propellant fracturing with a packer above the propellant charges.
Solution to the Problem. The present invention addresses this problem by placing restrictor plugs above and below the propellant charge with an outside diameter sized to deliver the appropriate amount of energy to fracture the surrounding formation without creating over-pressure conditions that might damage the well casing. The diameters of the restrictor plugs can be selected to allow more or less of the combustion gases to pass the restrictor plugs depending on specific well conditions.
For example, in an extremely impermeable formation with a high fracture extension pressure, the restrictor plug diameter can be reduced so that peak pressures would not exceed the casing burst pressure for extended durations, resulting in casing damage. In more permeable formations and those with lower fracture extension gradients, the restrictor plug diameter can be increased to achieve higher pressures, forcing more gas into the formation
The restrictor plugs are secured to a carrier which bears the opposing forces exerted by the combustion gases on the restrictor plugs. These forces tend to offset one another, so that the assembly tends to remain in the desired vertical position within the well. In addition, only negligible forces are exerted on the well casing adjacent to the restrictor plugs, unlike conventional packers.